Target structures such as are utilised in television transmission tubes



Jan. 10, 1956 c. A. JOHNSON 2,730,639

TARGET STRUCTURES SUCH AS ARE UTILISED IN TELEVISION TRANSMISSION TUBES Filed Oct. 16, 1950 F/G/ a 4 /fIVP/!70 CHARLES ALAN JOHNSON A-ffarney TARGET STRUCTURES SUCH AS ARE UTILISED IN TELEVISION TRANSMISSION TUBES Charles Alan Johnson, St. Margarets, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application October 16, 1950, Serial No. 190,293

4 Claims. (Cl. 313-67) This invention relates to target structures such as are utilised in television pick-up tubes, especially tubes of the kind which are adapted to operate with cathode potential stabilisation.

A known form of target structure utilised in television pick-up tubes comprises a sheet of insulating material to one side of which is applied a conductive signal plate and to the other side of which are applied a multiplicity of discrete photoemissive conductive, or so-called mosaic, elements, the surface of the insulating material being exposed at the latter side between said elements. In operation of a tube embodying such a target structure, a light image is projected on the mosaic elements and the elements become positively charged in dependence upon the brightness of elemental areas of the light image. If the television pick-up tube is arranged to operate with cathode potential stabilisation, the target structure is periodically scanned by a low velocity beam of electrons which, by supplying electrons to the mosaic elements, discharges said elements and restores them to a datum potential corresponding substantially to the potential of the cathode from which the beam of electrons originates.

It is found that when a tube embodying such a target structure is operated with cathode potential stabilisation, an undesirable eflect termed the photographic efiect is liable to arise. The effect manifests itself, for example, if the light image projected on the target structure is that of a dark object against a white background. After a short time the background, as observed in a reconstituted picture of the light image, becomes less white and if the dark object is removed the area of background now exposed appears whiter than the remainder and persists so for a short time, which may be of the order of several seconds, having the appearance of a negative image of the dark object. The eifect is believed due to the fact that areas of the insulating surface exposed between highly positively charged mosaic elements, that is (in the case described) the elements illuminated by the white background, become negatively charged as a result of the collection thereby of electrons. The collected electrons may be, for example, fast photo-electrons, or secondary electrons released by the scanning beam from the positively charged mosaic elements. The negative charge of these areas can leak ofi only very slowly and in effect temporarily desensitises the neighbouring mosaic elements by inhibiting, in the manner of a negatively biased control electrode, release of some of the photo-electrons which would otherwise be released from the mosaic elements.

The object of the present invention is to provide an improved target structure with a view to reducing the liability of said structure to give rise to the above-described photographic effect.

According to the present invention there is provided a target structure, such as is suitable for use in a television pick-up tube, comprising a layer of leaky dielectric material, a conductive electrode applied to one side of said layer, a multiplicity of discrete insulating elements United States Patent O applied to the other side of said layer, and photoemissive conductive elements on said insulating elements, said conductive elements not extending into contact with said layer.

The reference to a layer of leaky dielectric material herein and in the claims is intended to indicate a layer of dielectric material of such conductivity that electric charges on areas of the dielectric layer exposed between the insulating elements, which might otherwise accumulate on said areas, can leak through said layer to the conductive electrode in a short time of the order of M of a second, i. e. a frame period in British television practice, a practical upper limit for the time being about V: of a second. The liability for the photographic efiect to arise is reduced. At the same time loss of definition and sensitivity which would occur if substantial charge leakage were possible, respectively, from one conductive element to another and from the conductive elements to the conductive signal plate is restricted due to the insulating elements on which the conductive elements are provided.

In order that the said invention may be clearly understood and readily carried into efiect, the same will now be more fully described with reference to the accompanying drawing in which:

Figure 1 illustrates in transverse section a fragment of a target structure in accordance with one example of the present invention, and

Figures 2 and 3 illustrate stages in the manufacture of a target structure such as illustrated in Figure 1.

Referring to Figure 1, the target structure illustrated is such as might be employed in a television pick-up tube adapted to operate with cathode potential stabilisation. The target structure consists of a continuous sheet of leaky dielectric material 1, for example conducting glass, having applied to one side a continuous transparent metallic film 2, for example, of chromium. The metallic film 2 usually forms the signal plate of the tube in which the target structure is used. A multiplicity of discrete insulating dielectric elements 3, through which substantially no charge leakage can occur, are applied to the other side of the continuous dielectric sheet, the insulating elements being formed, for example, by lithium borate or possibly quartz. Conductive elements 4 of antimony are in turn applied to the exposed side of the insulating elements. Only two each of the elements 3 and 4 are indicated in the drawing for convenience of illustration. One antimony element 4 is provided on each insulating element 3, each antimony element being as indicated of a slightly smaller area than the respective insulating element 3 so that it does not extend into contact with the areas 5 of the dielectric sheet 1 exposed around the insulating elements. The target structure is mounted in the tube onwhich it is to be used in known manner, so that the side at which the antimony elements are exposed faces the electron gun of the tube, the tube being otherwise of normal construction.

The target structure described with reference to Figure 1, may be manufactured by the method described with reference to Figures 2 and 3. According to this method a metal mesh 6 having interstices 7 which are more open at one side of the mesh than at the other is employed. As shown, the mesh employed has filaments S of a substantially triangular cross-section, with the base of the triangle at one side of the mesh and the apex at the other side thereof. The mesh 6 is mounted with its more open side in close contact with the side 9 of the dielectric sheet 1 on which the insulating and conductive elements 3 and 4 are to be provided. Insulating material to form the insulating elements 3 is then evaporated through the interstices 7 of the mesh 6 on to the dielectric sheet and it is arranged that said material is evaporated from an extended source of sufficiently large extent that some of the evaporated particles have tra 1 through the intersticesj oft the meshtherefore extends to the areas of the dielectric sheet 1 which for normally incident particles Would;be. shadowed. by-the filaments 8 of the mesh 6. Theobliquity of the trajectories 10 is, however, insufiicient to cause insulating material deposited on the dielectric sheetl toeX-tend into contact with the sides of the filaments, whereby the discrete insulating; elements 3 which. are formed are each surrounded by an area of the, dielectric sheet. on which no insulating material is deposited. After the-insulating elements 3, have. been. deposited, andwit'nout moving the mesh antimony is evaporated asillustrated in Figure 3 through the interstices 7 froman-eflective point source which is sufiiciently removed from.the mesh that the evaporated particles of antimony: have trajectories which, as, indicated by dotted lines 11,.are; substantially normal to the dielectric sheet 1..- The. antimony evaporated through the interstices of the meshis-therefore deposited upon the previously deposited insulating elemental but does not extend into the shadowed; areas so that a discrete element .4 of antimony isformed .on each insulating element 3 but of smaller area than the insulating element so as not to extend into contact with .the dielectric sheet 1. Thereafter the meshis-removed. exposing the areas 5 of the dielectric sheet not covered by the. insulating elements. Subsequent processing of. the structure such as baking, and sensitisation of the antimony elements with caesium is efiected in the normal manner. The conductive film 2 to form. the signal plate maybe provided on the other side of the dielectric sheet 1 in any suitable manner either before or after the deposition of the insulating elements 3 andiconductive elements 4; It will also be appreciated that other: metals than antimony, for example bismuth or silver, may be. employed to form the photoemissive conductive elements.

The thickness of the dielectric is chosen to be such that in combination. with the insulating elements a suitable capacity is provided between. each conductive element and the signal plate, and the conductivity of the dielectric sheet is as aforesaid, arranged to. be suchthat any charge which accumulates on the. areasexposedbetween the insulating. elements canleak through the dielectric sheet to the signal plate. in a .shorttime, of the order of a frame period or a few frame periods. The area of insulating element exposed to the scanning beam around each conductive element is very small indeed so that negligible accumulation of charge should occur thereon.-

What- I claim is:

1. A target. structure, such. as, is suitable; for use. in a television pick-up tube, comprising a layer of leaky dielectric material, a conductive electrode applied to one side of said layer, a multiplicity of discrete insulating elements on the other side of 'said layer, and photoemissive conductive elements on said insulating elements, said conductive elements being spaced from said layer by said insulating elements.

2. A target structure, such asis suitable for use in a television pick-up tube, comprising a layer of leaky di- 3 electric" material, a continuous conductive layer on one surface'of said dielectric layer, a multiplicity of discrete insulating elements ontheother surface of said dielectric layer, and photoemissive conductive elements on and substantially. covering'said insulating elements, said conductive elements being spaced from said dielectric layer by said insulating elements, said leaky dielectric material having a conductivity proportioned for leakage to said conductive layer within one fifth second of electric charges collected by areas of said material exposed between saidinsulating elements, v H

3. In a television pick-up tube including means for producing a low velocityscanning beam of electrons, at target structure comprising a layer of leaky dielectric material having one surface turned to said means for producinga beam of electrons, a continuous conductive electrode on the other surface of said layer, a multiplicity of, discrete insulating elements on the first surface ofsaid layer, and photoemissive conductive elements on said; insulating elements, said conductive elements being spaced. from said layer by, said insulating elements, said leaky dielectricmaterial having a conductivity proportioned for lalrageto said conductive layer within one fifth second of electric chargcscollected by areas of said material exposed between said insulating elements.

4. A target structuresuch as is suitable for use in a television pick-up tube, comprising a layer of conductive glass, a continuous conductive layer on one surface of said first layer, a multiplicity of discrete insulating elements of a substance selected from the group consisting of lithium borate and quartz on the other surface of said first layer, and photoemissive conductive elements on and substantially covering said insulating elements, said conductive elements beingspaced from said first layer by said insulating elements.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain Mar. 15, 1950 

